1. Field of the Invention
This invention relates to a distortion compensation circuit for compensating distortion in a power amplifier and particularly to a distortion compensation circuit with so-called Memory-effect and to a transmission apparatus to which the distortion compensation circuit is applicable.
2. Description of the Related Art
Recently, in cellular phone systems, new systems have been developed such as a W-CDMA (Wideband-Code Division Multiple Access), CDMA (Code Division Multiple Access) 2000, which are called as the third generation system. In these cellular phone systems, an occupied bandwidth per one channel becomes considerably wider than that in a conventional PDC (Personal Digital Cellular) and PHS (Personal Handyphone System: Trademark). More specifically, the above-described occupied bandwidth per one channel is, for example, in the PDC, about 20 KHz and, in the PHS, it is about 200 KHz. Further, the occupied bandwidth in the CDMA 2000 is about 1.2 MHz and that in the W-CDMA becomes about 4 MHz.
Here, if a signal having a wide bandwidth, such as signals handled in the CDMA 2000 and the W-CDMA, is amplified, it becomes difficult to keep the characteristics of the amplifier within the band flat. Particularly, since requirements for distortion in a power amplifier (PA) is severe, designing of such power amplifier tends to be difficult. On the other hand, the distortion compensation technology is useful for improving distortion in a power amplifier and improving the efficiency thereof. However, it is difficult to efficiently compensate distortion in power amplifiers having a large deviation over its band. Thus, there is no appropriate method of compensating distortion in such power amplifier.
On the other hand, as an example of technology for compensating distortion in a power amplifier having the so-called Memory-effect, a Predistortion method is disclosed in which a predistortion signal is generated in a baseband section with a DSP (Digital Signal Processor). For example, J. Kim and K. Konstantinou “Digital predistortion of wideband signals based on power amplifier model with memory”, IEEE Electronics letters, 8th Nov. 2001, Vol. 37, No. 23, pp. 1417-1418 discloses this.
According to the technology disclosed in this document, a signal voltage at a sample point is compared with a signal voltage at a previous sample point by one sample with regard to a baseband signal sequence, and a predistortion signal generation section generates a predistortion signal on the basis of the comparison result.
However, in this method of generating a predistortion signal, because the predistortion signal is generated at the baseband section, it is difficult to compensate distortion over the entire bandwidth of the power amplifier. In addition, because the predistortion signal is generated by comparison with only one-sampling previous value, this method has no effect on power amplifiers showing the strong memory effect.